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When the time comes to deploy the fridge-sized DART spacecraft, it will be travelling at nine times the speed of a bullet in an attempt to knock one of the two asteroids - named Didymos A and B - off its trajectory.

“A binary asteroid is the perfect natural laboratory for this test,” said Tom Statler, program scientist for DART at NASA Headquarters.

“The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn’t change the orbit of the pair around the sun.”

While small asteroids hit the Earth every day, larger ones like the Didymos twins could cause real problems if they hit us. And this is why NASA wants to use them as target practice.

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Using an on-board targeting system, the DART would fly itself to Didymos B and smash into it at 3.7 miles-per-second in an impact that will be visible from Earth-based observatories.

The kinetic impact technique works by changing the speed of a threatening asteroid by a small fraction of its total velocity, but by doing it well before the predicted impact so that this small nudge will add up over time to a big shift of the asteroid’s path away from Earth.

"DART is a critical step in demonstrating we can protect our planet from a future asteroid impact," said Andy Cheng of The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland and co-lead of the DART investigation project.